Display apparatus and driving device for displaying

ABSTRACT

A display driver having a first mode where a number of colors to be displayed is smaller than a predetermined number and a second mode where a number of colors to be displayed is equal to or larger than the predetermined number. A generation circuit generates gray scale voltages having a plurality of levels for positive polarity and negative polarity. In the first mode, the generation circuit decreases a current flowing into an internal circuit which generates a gray scale voltage of an intermediate level other than two gray scale voltages of a lowest and a highest level, in comparison to a current flowing in the second mode. In the first mode, the selection circuit selects a gray scale voltage in accordance with the display data, from the two gray scale voltages of the lowest and the highest level gray scale voltage which are generated from the generation circuit.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 11/776,158, filedJul. 11, 2007, which is a continuation of U.S. application Ser. No.10/918,379, filed Aug. 16, 2004 (now U.S. Pat. No. 7,450,099), which isa continuation of U.S. application Ser. No. 10/161,638, filed Jun. 5,2002 (now U.S. Pat. No. 6,781,605), which relates to U.S. applicationSer. No. 11/157,063, filed Jun. 21, 2005 (now U.S. Pat. No. 7,227,560).This application relates to and claims priority from Japanese PatentApplication No. 2001-171889, filed on Jun. 7, 2001. The entirety of thecontents and subject matter of all of the above is incorporated hereinby reference.

BACKGROUND OF THE INVENTION

The present invention relates to a display apparatus having a displaypanel in which display pixels are arranged in matrix and a drivingdevice for displaying adapted to deliver the display panel gray scalevoltages matching with display data and more particularly to, a displayapparatus using liquid crystal, organic EL, plasma or the like and adriving device for displaying adapted for use in the same.

JP-A-6-348235 discloses a liquid crystal panel having X-signal lines andY-signal lines, a horizontal driver for selecting, on the basis of adata signal of an image to be displayed, one gray scale signal from aplurality of gray scale signals deliver out of a gray scale voltagegenerating circuit and delivering the selected gray scale voltage to anX-signal line of the liquid crystal panel, and a vertical driver fordelivering a scanning signal of the liquid crystal panel to a Y-signalline of the liquid crystal panel. It further discloses that the grayscale voltage generating circuit has a plurality of fixed resistorsconnected in series between a high-potential reference voltage and alow-potential reference voltage and voltage variable means for varyingvoltages at connection nodes of the fixed resistors between thehigh-potential and low-potential reference voltages. It furtherdiscloses that voltages at connection nodes of the fixed resistors aredelivered as gray scale signals. It further discloses that the voltagevariable means includes a variable resistor connected between thehigh-potential and low-potential reference voltages and an operationalamplifier having its one input terminal connected to a voltage variableterminal of the variable resistor and its output terminal connected to apredetermined connection node of the plurality of fixed resistors.

JP-A-10-142582 discloses that a resistor ladder circuit comprised ofresistors R0, R1, . . . , Rn and a resistor ladder circuit comprised ofresistors R0′, R1′, . . . , Rn′ are connected between a power supplyterminal Vcc and a connection terminal GND, operational amplifiers OP1,OP2, . . . , OPn are connected between the two resistor ladder circuits,and a constant voltage generating circuit comprised of an operationalamplifier OP0 is connected to a node of the resistors Rn−1′ and Rn′. Itfurther discloses that output currents of the operational amplifiers OPnand OP1 are adjusted by the resistors Rn and R1 and an output current ofthe operational amplifier OP0 is adjusted by the resistor Rn′.

JP-A-2001-22325 discloses a liquid crystal display apparatus having avoltage division circuit for generating a plurality of positive-negativesymmetrical reference voltages from positive and negative referencevoltages, a variable voltage generating circuit for supplyingpositive-negative symmetrical reference voltages for gray scaleadjustment to one pair of positive-negative symmetrical voltage divisionpoints corresponding to a specified halftone of the voltage divisioncircuit, and one pair of amplifiers. It further discloses a liquidcrystal display apparatus having a voltage division circuit forgenerating a plurality of positive-negative symmetrical referencevoltages from positive and negative reference voltages, a plurality ofvariable voltage generating circuit for supplying positive-negativesymmetrical reference voltages for gray scale adjustment to a pluralityof pairs of positive-negative symmetrical voltage division pointscorresponding to a plurality of specified halftones of the voltagedivision circuit, and a plurality of differential amplifiers.

But none of the above prior arts take the following problems intoaccount.

Firstly, at the output of the gray scale voltage generating circuit,capacitive components including liquid crystal are charged/dischargedsimultaneously at a timing of A.C. (Alternating Current) operation. Forthe purpose of compensating the phase for an abrupt current change dueto the charge/discharge, it is general to connect a stabilizingcapacitor of 0.1 to 10 μF. to the output of a voltage follower circuit.The stabilizing capacitor having a large capacitance as above needs tobe provided externally of an IC even when the gray scale voltagegenerating circuit is implemented in an IC form, thus increasing thenumber of parts. Secondly, in the liquid crystal display apparatus, withthe aim of preventing an image quality degradation similar to print,there needs a so-called A.C. operation for inverting, at a constantperiod, the polarity of a voltage applied to liquid crystal. In thisphase, through a so-called asymmetric drive method for instance thatmakes individual levels of gradation voltages different at the positiveand negative polarities, the amplitude of common voltage can be reducedand an image quality degradation such as flicker can be prevented toadvantage. To realize the asymmetric drive with the gray scale voltagegenerating circuit, the level of reference voltage may be different forthe positive and negative polarities. But, with the level of referencevoltage changed periodically, the stabilizing capacitor ischarged/discharged and as a result, consumption power increases.

Thirdly, in the liquid crystal display apparatus, an instance issupposed in which the number of colors possessed by input display datais smaller than the number of levels of generated gray scale voltages.In that case, unneeded gray scale voltages are generated by means of aladder resistor, with the result that a consumptive steady current flowsthrough the ladder resistor.

SUMMARY OF THE INVENTION

A first object of the invention is to provide a display apparatuscapable of reducing the number of parts by providing a voltage followercircuit dispensing with a stabilizing capacitor and to provide itsdriving device for displaying.

A second object of the invention is to provide a display apparatuscapable of avoiding an increase in consumption power by suppressingcharge/discharge current of a stabilizing capacitor during asymmetricdrive and to provide its driving device for displaying.

A third object of the invention is to provide a display apparatuscapable of suppressing an increase in consumption power by eliminating awaste of steady current flowing through a ladder resistor in match withthe number of display colors and to provide its driving device fordisplaying.

When the stabilizing capacitor is supposedly removed in the generalvoltage follower circuit, the most critical problem is that the phasemargin decreases, giving rise to a tendency to oscillation. To obviatethis problem, insertion of a resistor between the output of the voltagefollower circuit and a load is effective.

Accordingly, to accomplish the first object, a voltage follower circuitaccording to an aspect of the invention comprises a differentialamplifier, first and second buffer circuits, a resistor and twocompensating capacitors, wherein outputs of the first and second buffercircuits are connected to each other through a resistor, the output ofthe first buffer circuit is fed back to one input of the differentialamplifier and the output of the second buffer circuit serves as anoutput of the voltage follower circuit. By providing the resistorbetween the output of voltage follower circuit and a feedback point ofthe differential amplifier in this manner, the phase margin can beincreased and output operation can be stabilized. At the same time, theoutput of the second buffer circuit can directly drive the load, therebyensuring that the time constant can be decreased and the recovery timefrom voltage variations can be shortened.

To accomplish the second object, in a display apparatus according toanother embodiment of the invention, voltage follower circuits forpositive and negative polarities are provided and they are switched.With this construction, a potential applied to the stabilizing capacitorcan be constant even in asymmetric drive of the stabilizing capacitor tothereby suppress the charge/discharge.

To accomplish the third object, in a display apparatus according tostill another aspect of the invention, switches are provided whichseparate a ladder resistor at portions for generating unnecessary grayscale voltages or switch the ladder resistor to a high-resistanceresistor. With this construction, steady current flowing through theladder resistor can be optimized in match with the number of colorspossessed by display data to avoid an increase in consumption power.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the construction of a voltage followercircuit according to a first embodiment of the invention.

FIG. 2 is a circuit diagram showing the detailed construction of anexample of the voltage follower circuit in the first embodiment of theinvention.

FIG. 3 is a circuit diagram showing the detailed construction of anotherexample of the voltage follower circuit in the first embodiment of theinvention.

FIG. 4 is a circuit diagram showing the construction of a gray scalevoltage generating circuit in the first embodiment of the invention.

FIG. 5 is a block diagram showing the construction of a drain drivecircuit in the first embodiment of the invention.

FIG. 6 is a diagram useful to explain operation of the drain drivecircuit in the first embodiment of the invention.

FIG. 7 is a timing chart showing operation of the drain drive circuit inthe first embodiment of the invention.

FIG. 8 is a circuit diagram showing the construction of a gray scalevoltage generating circuit according to a second embodiment of theinvention.

FIG. 9 is a circuit diagram showing the construction of a gray scalevoltage generating circuit according to a third embodiment of theinvention.

FIG. 10 is a circuit diagram showing the construction of a gray scalevoltage generating circuit according to a fourth embodiment of theinvention.

FIG. 11 is a circuit diagram showing the construction of a gray scalevoltage generating circuit according to a fifth embodiment of theinvention.

FIG. 12 is a block diagram of an information processing apparatusprovided with the gray scale voltage generating circuit according to theinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 illustrates the fundamental construction of a voltage followercircuit according to a first embodiment of the invention. The voltagefollower circuit, as designated at reference numeral 101, in the presentembodiment comprises a differential amplifier 102, first and secondbuffer circuits 103 and 104, a resistor 105 and two compensatingcapacitances (for example, capacitors) 106 and 107. This circuitconstruction is featured in that outputs of the first and second buffercircuits 103 and 104 are connected to each other through the resistor105, with the output of first buffer circuit 103 being fed back to oneinput of the differential amplifier 102 and the output of the secondbuffer circuit 104 being connected to serve as an output of the voltagefollower circuit 101. By providing the resistor 105 between the outputof voltage follower circuit 101 and a node for feedback to thedifferential amplifier 102 in this manner, the phase margin can beincreased to stabilize output operation. In addition, since the outputof second buffer circuit 104 can directly drive a load, the timeconstant can be reduced and the recovery time from voltage variationscan be shortened. In other words, the first buffer circuit 103 has ahigh phase margin to function to prevent oscillation. The second buffercircuit 104 has a high through rate to function to reduce the feedbacktime.

The differential amplifier 102 receives at the other input an input fromthe outside of the voltage follower circuit 101 and, at one input, thefed back input. The output of differential amplifier 102 connects toground through the compensating capacitor 107. The first and secondbuffer circuits 103 and 104 receive the output of the differentialamplifier 102. The compensating capacitor 106 is connected in parallelwith the first buffer circuit 103. The output of the second buffercircuit 104 serves as the output of the voltage follower circuit 101.The output of first buffer circuit 103 and the output of second buffercircuit 104 routing through the resistor 105 are fed back to thedifferential amplifier 102.

The first embodiment of the invention will now be described withreference to FIGS. 2 to 7. The first embodiment of the invention isdirected to a voltage follower circuit dispensing with theaforementioned stabilizing capacitor, the detailed circuit constructionof which is exemplified in FIGS. 2 and 3.

In FIG. 2, a voltage follower circuit 201 comprises MOS transistors 202to 211, a resistor 212 and compensating capacitances (for example,capacitors) 213 and 214. For the correspondence with components of FIG.1, it will be appreciated that the differential amplifier 102corresponds to a component constituted by the MOS transistors 202 to205, the first buffer circuit 103 corresponds to a component constitutedby the MOS transistor 206 and the second buffer circuit 104 correspondsto a component constituted by the MOS transistors 207 to 211.

A voltage follower circuit 301 shown in FIG. 3 differs from the voltagefollower circuit 201 shown in FIG. 2 in that interchange of P channelwith N channel of the MOS transistors and interchange of connection ofpower supply voltage are carried out.

When considering the range of available voltage level, the voltagefollower circuit 201 has the output range close to ground voltage GND(low voltage) and the voltage follower circuit 301 has the output rangeclose to power supply voltage VDV (high voltage). Accordingly, it isdesirable that the two circuits be used purposively in accordance withthe level of a gray scale voltage to be delivered. A voltage followercircuit with stabilizing capacitor based on a so-called class “A”amplifier has a wider output range than that of the voltage followercircuit of the present invention. Therefore, for only a gray scalevoltage at a level close to the power supply voltage VDD or groundvoltage GND, the voltage follower circuit with stabilizing capacitor maybe used.

An example of the circuit construction based on this idea and used forgenerating 64 kinds of gray scale voltages will be described.

FIG. 4 is a circuit diagram showing the construction of a gray scalevoltage generating circuit 401. Comprises voltage follower circuits withstabilizing capacitors 402, 403 and 408, 409 based on so-called class“A” amplifiers, voltage follower circuits 404, 405 each implemented withthe aforementioned voltage follower circuit 301 according to the presentembodiment and voltage follower circuits 406, 407 each implemented withthe aforementioned voltage follower circuit 201 also according to thepresent embodiment. As will be seen from FIG. 4, the voltage followercircuits 402 to 409 are arranged in such a manner that the levels ofvoltages delivered out of these circuits become lower in this order.More particularly, the voltage level gradually decreases from thevoltage follower circuit 402 to the voltage follower circuit 409. Aladder resistor 410 divides outputs of the voltage follower circuits 402to 409 to generate gray scale voltages V0 to V63. In the gray scalevoltage generating circuit 401 of the present invention, referencevoltages, generally designated at Vref, are not inputted externally butare generated by inputting a 2-level reference voltage of referencevoltage VH at a high level and reference voltage VL at a low level anddividing the 2-level reference voltage by a ladder resistor 411.Advantageously, this can reduce the number of wiring lines leading tothe outside. With the circuit construction described above, the grayscale voltage generating circuit 401 can generate 64 kinds of gray scalevoltages. In each of the voltage follower circuits with stabilizingcapacitor 402, 403, 408 and 409, the differential amplifier has itsoutput fed back to its input. The output of the differential amplifieris connected on the other hand to ground through a stabilizingcapacitance 412 (for example, capacitor).

Next, construction and operation of a drain drive circuit including thegray scale voltage generating circuit 401 will be described by taking aninstance where so-called Vcom modulation drive is carried out.

FIG. 5 is a block diagram of the drain drive circuit designated at 501.The circuit 501 comprises a data latch circuit 502 for latching displaydata for one line, a data inversion circuit 503 for inverting thepolarity of the display data, and gray scale voltage selecting circuits504 each adapted to select a gray scale voltage matching with the datafrom a plurality of gray scale voltages V0 to V63 generated by the grayscale voltage generating circuit 401. Firstly, the drain drive circuit501 receives from an external liquid crystal controller a signal of CL1indicative of one scanning period (one horizontal period), a signal ofEN indicative of a period for transfer of effective display data, asignal of M indicative of the polarity of alternation (making italternate), a signal of CL2 indicative of a transfer clock of thedisplay data and a signal of DATA indicative of the display data. In thepresent embodiment, it is assumed that the DATA has gray scaleinformation of plural bits (for example, 6 bits) in respect of eachpixel.

FIG. 6 shows an example of operation of the drain drive circuit 501. Thedata latch circuit 502 repeats an operation in which it stores DATA forone line during a period of the EN being high (=1) by using the CL2 asfetching clock and delivers, as LD (line data), the stored DATA at atime in synchronism with the CL1. The data inversion circuit 503receives inputs of the LD and M and delivers a PD while keeping the LDunchanged when the M is at low level (=0) or after inverting the LD whenthe M is at high level (=1). Each of the gray scale voltage selectingcircuits 504 selects one gray scale voltage from the inputted gray scalevoltages V0 to V63 in accordance with a value of the PD so as to delivera VD.

FIG. 7 shows a timing chart useful to sum up the operation of the draindrive circuit 501. As will be seen from FIG. 7, voltage levels matchingwith the display data are delivered in accordance with the CL1, thusrealizing driving waveforms of general Vcom modulation drive.

The drain drive circuit 501 described herein was made in the form of anIC and actual characteristics were measured. Firstly, as for the outputrange, the voltage follower circuits 404 and 405 of the invention had(VDD−0.6V) or less and the voltage follower circuit 406 and 407 of theembodiment had (GND+0.8V) or more. In addition, when a 2-inch TFT liquidcrystal device of 120×160 pixels was connected to the drive circuit tocarry out the Vcom modulation drive at a frame frequency of 60 Hz, allgray scale voltages can be freed from adversity such as oscillation andan excellent display can be obtained.

As will be seen from the above, the voltage follower circuits 404 to 407of the present embodiment can provide good characteristics even withoutresort to the stabilizing capacitor 412 and therefore, in comparisonwith the conventional drain drive circuit, can reduce the number ofparts of stabilizing capacitor. In the gray scale voltage generatingcircuit 401 in the present embodiment, the voltage follower circuitswith stabilizing capacitor are used in combination but this constructionis not limitative. For example, if the ranges close to the power supplyvoltage VDD and the ground voltage GND are not used, the circuit may beconstructed of only the voltage follower circuits 201 and 301 in thepresent embodiment.

Next, other embodiments of the invention will be described withreference to FIGS. 8 to 10. In other embodiments, gray scale voltagegenerating circuits are provided which can realize suppression ofcharge/discharge of a stabilizing capacitor during asymmetrical drive.Like the foregoing, voltage follower circuits for positive and negativepolarities are provided and they are used in switching fashion.

FIG. 8 illustrates a circuit diagram showing the construction of a grayscale voltage generating circuit according to a second embodiment of theinvention. The circuit comprises switches for switching gray scalevoltages for positive and negative polarities, a ladder resistor 803 forgenerating gray scale voltages for positive polarity, and a ladderresistor 804 for generating gray scale voltages for negative polarity.Other components are identical to those of the gray scale voltagegenerating circuit 401 shown in FIG. 4. This circuit construction isfeatured in that there are provided two kinds of ladder resistors 803and 804 for positive and negative polarities and two kinds of voltagefollower circuits 402 and 408 for positive polarity and voltage followercircuits 403 and 409 for negative polarity and that switches areprovided which respond to an A.C. signal from the liquid crystalcontroller to switch the two kinds of ladder resistors and the two kindsof voltage follower circuits. In connection with the voltage followercircuits 402, 403 and 408, 409, outputs of these circuits are switchedby means of the switches. The switches 801 succeed the voltage followercircuits with stabilizing capacitors 402, 403 and 408, 409 whereas theswitches 802 precede voltage follower circuits without stabilizingcapacitors 404 to 407 in this embodiment. For example, during a periodfor the A.C. signal to be low (=0), the switch 801 selects a referencevoltage obtained by dividing a potential difference between referencevoltages VHP and VLP by means of the ladder resistor 803 for positivepolarity and during a period for the A.C. signal to be high (=1),selects a reference voltage obtained by dividing a potential differencebetween reference voltages VHN and VLN by means of the ladder resistor804 for negative polarity. With this construction, the outputs of thevoltage follower circuits 402, 403 and 408, 409 can be unvarying andcharge/discharge of the stabilizing capacitor 412 can be avoided. On theother hand, for the voltage follower circuits 404 to 407 dispensing withstabilizing capacitor in this embodiment, the switches 802 precedeamplifier inputs. This is because the number of amplifiers to be usedcan be decreased in comparison with the construction in which theamplifier input is followed by the switch. Like the foregoing, during aperiod for the A.C. signal to be low (=0), the switch 802 selects areference voltage obtained by dividing a potential difference betweenreference voltages VHP and VLP by means of the ladder resistor 803 forpositive polarity and during a period for the A.C. signal to be high(=1), selects a reference voltage obtained by dividing a potentialdifference between reference voltages VHN and VLN by means of the ladderresistor 804 for negative polarity.

FIG. 9 illustrates a circuit diagram showing the construction of a grayscale voltage generating circuit according to a third embodiment of theinvention. The circuit comprises switches 901 each adapted toselectively switch connection between a voltage follower circuit and twostabilizing capacitors 412 and switches 902 each adapted to make achoice of a reference voltage generated by a ladder resistor forpositive polarity or a reference voltage generated by a ladder resistorfor negative polarity. It will be seen from FIG. 9 that with thiscircuit construction, only the stabilizing capacitors 412 are providedfor positive and negative polarities and the switch is provided whichselectively switches connection of each of the voltage follower circuits402, 403 and 408, 409 to each of the stabilizing capacitors. Thisconstruction is advantageous over the gray scale voltage generatingcircuit in the second embodiment shown in FIG. 8 in that the number ofthe voltage follower circuits 402, 403 and 408, 409 can be one for onereference voltage to reduce the circuit scale. The switches 902 precedethe voltage follower circuits. The switch 901 selects one, for positivepolarity, of the two stabilizing capacitors 412 during a period for theA.C. signal to be low (=0) and selects the other, for negative polarity,of the two stabilizing capacitors during a period for the A.C. signal tobe high (=1).

FIG. 10 illustrates a circuit diagram showing the construction of a grayscale voltage generating circuit according to a fourth embodiment of theinvention. The circuit comprises resistors 1001 for positive polarity,resistors 1002 for negative polarity, and switches 1003 each adapted toselectively switch connection between a ladder resistor for generationof reference voltages and each of the resistors 1001 and 1002 forpositive and negative polarities. The circuit construction shown in FIG.10 intends to singularize the ladder resistor for generation ofreference voltages. Specifically, any one of the ladder resistors 803and 804 in the gray scale voltage generating circuit shown in FIG. 8 canbe unneeded. Namely, by making resistance of the resistor 1001 differentfrom that of the resistor 1002, gray scale voltages at different levelscan be generated at positive and negative polarities. This constructionis preferably applied to upper and lower ends of the ladder resistor soas to enhance the degree of freedom of adjustment. The switch 1003selects the resistor 1001 for positive polarity during a period for theA.C. signal to be low (=0) and selects the resistor 1002 for negativepolarity during a period for the A.C. signal to be high (=1).

In the gray scale voltage generating circuits according to the second tofourth embodiments of the invention set forth so far, even when theso-called asymmetric drive is carried out, charge/discharge of thestabilizing capacitor connected to the voltage follower circuit can beavoided. Accordingly, a drain drive circuit of more reduced consumptionpower can be provided.

FIG. 11 illustrates a circuit diagram showing the construction of a grayscale voltage generating circuit according to a fifth embodiment of theinvention. In the fifth embodiment of the invention, a method isdisclosed which can optimize steady current flowing through a ladderresistor in match with the number of colors possessed by display datawith the aim of eliminating any consumptive steady current flowing thethrough the ladder resistor. In the fifth embodiment, color numberinformation possessed by display data is of multiple bits (for example,plural bits amounting to 6 bits) or is of minor bits (for example, asingle bit amounting to one bit) and it is assumed that either state isdetermined by information supplied from the external liquid crystalcontroller. It is also assumed that when the color number informationpossessed by the display data is of 6 bits, all of 64 kinds of grayscale voltages (V0 to V63) are used and in case of one bit, only grayscale voltages at opposite ends (V0 and V63) are used.

The circuit of FIG. 11 comprises resistors 1101 for 6 bits, resistors1102 for one bit, and switches 1103 each adapted to switch connection tothe resistor 1101 for 6 bits and connection to the resistor 1102 for onebit in accordance with the color number information. For example, whenthe color number information is high (=1), the switch 1103 recognizes a6-bit display mode to select the resistor 1101 for 6 bits and when thecolor number information is low (=0), it recognizes a one-bit displaymode to select the resistor 1102 for one bit. Here, the resistance ofthe one-bit display resistor 1102 is so determined in advance as to besufficiently larger than that of the 6-bit resistor 1101 in order thatwhen the resistor 1102 is selected, the current flowing through theladder resistor can be reduced. This takes advantage of the fact that inthe display mode of the color number being one bit, only the gray scalevoltages at opposite ends (V0 and V63) are used as described previouslyand consequently, even if the levels of other gray scale voltages vary,this variation does not affect display. In case the display data is ofminor bits, the number of gray scales of the display data is small butin case the display data is of multiple bits, the number of gray scalesof the display data is large. In place of the resistor 1101 for 6 bits,resistor 1102 for one bit and switch 1103, a variable resistor can beused and in place of the switch 1103, a variable resistor controlcircuit can be used. For example, if the color number information ishigh (=1), the variable resistor control circuit recognizes the 6-bitdisplay mode to decrease the resistance of the variable resistor and ifthe color number information is low (=0), it recognizes the one-bitdisplay mode to increase the resistance of the variable resistor.

As described above, the gray scale voltage generating circuit accordingto the fifth embodiment of the invention can control the value ofcurrent flowing through the ladder resistor for generation of gray scalevoltages in accordance with the color number information. Accordingly, adrain drive circuit of more reduced consumption power can be provided.In the fifth embodiment of the invention, two kinds of resistors for 6bits and one bit are provided but this is not limitative and forexample, in the one-bit display mode, V0 and V63 can be completelydisconnected from the ladder resistor.

Thus, in an application of the gray scale voltage generating circuitaccording to the fifth embodiment of the invention to a handy phone oran information processing apparatus requiring low power such as PDA,when the information processing apparatus is in waiting condition, theMPU of the information processing apparatus may command informationpurporting that the number of colors is small to the gray scale voltagegenerating circuit and when the information processing apparatus is incall condition (during communication), the MPU of the informationprocessing apparatus may command information purporting that the numberof colors is large to the gray scale voltage generating circuit. Inother words, the information processing apparatus gives a display ofmultiple gray scale when the user makes use of it but gives a display ofminor gray scale when the user does not make use of it. Through this,power consumption in the gray scale voltage generating circuit can bereduced when the user does not make use of it, thus reducing powerconsumption in the information processing apparatus.

FIG. 12 illustrates a block diagram of an information processingapparatus provided with the gray scale voltage generating circuitaccording to the invention. The information processing apparatus, asgenerally designated at reference numeral 1, comprises a displayapparatus 2 for displaying display data and a MPU 3 for performingoperation processing. The display apparatus 2 includes a display panel 4having display pixels arranged in matrix, a drain drive circuit 501 forgenerating gray scale voltages corresponding to the display data andapplying the gray scale voltages to the display panel 4, a gate drivecircuit 5 for selecting a line of pixels to be applied with the grayscale voltages, and a system power supply generating circuit 6 forgenerating operating power supply for the drain drive circuit 501 andgate drive circuit 5. The system power supply generating circuit 6generates reference voltages VH, VL, VHP, VLH, VHN and VLN and a powersupply voltage VDD for the drain drive circuit 501. The drain drivecircuit 501 includes a gray scale voltage generating circuit 401 forgenerating a plurality of gray scale voltages V0 to V63, a gray scalevoltage selecting circuit 504 for selecting one gray scale voltagematched with the display data from the plurality of gray scale voltagesV0 to V63, a system interface 505 for receiving the display data andcontrol signals from the MPU 3, a display memory 506 (for example, RAM)for temporarily storing one frame of the display data, and a controlregister 507 for setting gray scale voltage characteristics matchingwith characteristics of the display panel 4. The control register 507includes a register for adjusting the amplitude, a register foradjusting the gradient and a register for performing fine adjustment, inthe relation between gray scale number and gray scale voltage.

The first to fifth embodiments of the invention have been described byway of example of the Vcom modulation drive but this is not limitativeand the invention may also be applied, on the basis of a similar idea,to dot inversion drive and column inversion drive known as other drivemethods.

Advantageously, the drain drive circuit according to the invention canreduce the number of external stabilizing capacitors used or dispensewith the external stabilizing capacitor to thereby attain costreduction. Further, even when external stabilizing capacitors areprovided, the circuit can be so constructed as to prevent thestabilizing capacitor per se from being charged/discharged andconsequently, consumption power can be reduced. Further, the necessarysteady current can be controlled in match with the number of colorspossessed by input display data, thereby making it possible to furtherreduce consumption power.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A display driver comprising: a generation circuit that generates gray scale voltages of a plurality of levels; and a selection circuit that selects a gray scale voltage in accordance with a display data among the gray scale voltages generated by said generation circuit, wherein said display driver has a first mode in which a number of colors to be displayed on a display panel is smaller than a predetermined number, and a second mode in which the number of colors to be displayed on the display panel is equal to or larger than the predetermined number, wherein said generation circuit generates gray scale voltages having a plurality of levels for positive polarity and gray scale voltages having a plurality of levels for negative polarity, wherein, in the first mode, said generation circuit decreases a current flowing into an internal circuit which generates a gray scale voltage of an intermediate level other than two gray scale voltages of a gray scale voltage of a lowest level and a gray scale voltage of a highest level among the gray scale voltages of the plurality of levels, in comparison to a current flowing in the second mode, and wherein, in the first mode, said selection circuit selects a gray scale voltage in accordance with the display data, from the two gray scale voltages of the lowest level gray scale voltage and the highest level gray scale voltage which are generated from said generation circuit.
 2. The display driver according to claim 1, further comprising: a switching circuit that controls a resistance of a first resistor to divide a voltage between the lowest level gray scale voltage and the intermediate level gray scale voltage, and controls a resistance of a second resistor to divide a voltage between the highest level gray scale voltage and the intermediate level gray scale voltage with each other, wherein, in the first mode, said switching circuit increases the resistance of the first resistor and the resistance of the second resistor.
 3. The display driver according to claim 2, wherein said first resistor includes a third resistor and a fourth resistor having a lower resistance than a resistance of said third resistor, said third and fourth resistors being connected in parallel with each other, wherein said second resistor includes a fifth resistor and a sixth resistor having a lower resistance than a resistance of said fifth resistor, said fifth and sixth resistors being connected in parallel with each other, and wherein, in the first mode, said switching circuit selects the third resistor and the fifth.
 4. The display driver according to claim 2, wherein both said first and second resistors are configured to be variable resistors, and wherein, in the first mode, said switching circuit varies the resistance of said first and second resistors so as to increase the resistance of both said first and second resistors.
 5. The display driver according to claim 1, wherein said generation circuit decreases the current flowing into said internal circuit using a resistor.
 6. The display driver according to claim 1, wherein said generation circuit decreases the current flowing into said internal circuit by controlling the resistance of at least one of said first and second resistors in accordance with a selected mode between said first and second modes.
 7. The display driver according to claim 6, wherein, in the second mode, said selection circuit selects a gray scale voltage in accordance with the display data from gray scale voltages of a plurality of levels including the lowest level gray scale voltage, the highest level gray scale voltage, and the intermediate level gray scale voltage, which are generated from said generation circuit, and wherein said display driver includes a circuit to switch the current flowing into said internal circuit in accordance with the selected mode.
 8. The display driver according to claim 1, wherein said internal circuit includes a dividing circuit, and wherein said dividing circuit generates gray scale voltages of a plurality of levels including the highest level gray scale voltage, the lowest level gray scale voltage, and the intermediate level gay scale voltage, by dividing a reference voltage.
 9. The display driver according to claim 1, wherein said dividing circuit is formed of a plurality of resistors.
 10. The display driver according to claim 1, wherein information representing either of the first and second modes is inputted from an external device of said display driver. 